1. Field of the Invention
The present invention relates generally to semiconductor devices and, more particularly, to a high-speed transistor device with a hetero-junction structure.
2. Description of the Related Art
Conventionally, a high-electron mobility transistor, called the "HEMT" has been well known among those skilled in the art as a semiconductor field effect transistors capable of executing high-speed switching operations by utilizing a hetero-junction structure. Such type of transistor may be defined as the field effect transistor which is comprised of gallium arsenide and gallium aluminum arsenide, with a Schottky metal contact on the gallium aluminum arsenide layer and two ohmic contacts penetrating into the gallium arsenide layer, serving as the gate, source and drain respectively.
In the HEMT, what is called the "two-dimensional electron gas" spreads inside a non-doped gallium arsenide layer functioning as the channel region, to thereby attain high-speed transistor operations. The electron mobility of the HEMT can further improved by using the gallium arsenide layer as a channel region, because of its high electron mobility. As a result, extra-high-speed switching operations can be accomplished for the HEMT.
However, the gallium-arsenide high-electron mobility materials come with a serious technical problem: It is very difficult to make a gate insulation layer of high quality, which has been easily obtained in the metal oxide semiconductor field effect transistors (MOSFETs) using one of the most popular semiconductor materials, i.e., silicon. Due to such difficulty in manufacture of the gate insulation layer, HEMTs are not manufactured successfully using the presently available manufacturing technology, having the insulated-gate (insulated gate) type. For these reasons, development of a very high switching speed transistor is difficult.